Pulse domain neuromorphic integrated circuit for computing motion

ABSTRACT

An integrated circuit that computes the velocity of a visual stimulus moving between two photoreceptor locations is disclosed. In its most basic version, the circuit comprises two temporal edge detectors with photoreceptors, two pulse-shaping circuits, and one motion circuit on a single silicon chip. Velocity is computed from the signed time delay of the appearance of an image feature at the two photoreceptor locations. Specifically, each temporal edge detector detects a rapid irradiance transient at its photoreceptor location and converts it into a short current spike. This current spike is transformed into two different voltage pulses, a fast pulse and a slowly-decaying pulse, by the pulse-shaping circuit that is coupled to the temporal edge detector. The slowly-decaying voltage pulse produced at one location together with the fast voltage pulse generated at the other location, act as inputs to the motion circuit which generates a signal representative of the speed of motion for one sign or direction of motion. A pair of motion circuits encodes velocity, each motion circuit encoding speed for one of the two opposing directions of motion. The motion circuits are sample-and-hold circuits that use the fast pulse from one location to sample the slowly-decaying pulse from the other location. The individual motion-sensing cells are compact, and are therefore suited for use in dense one-dimensional or two-dimensional imaging arrays. Various embodiments are described.

BACKGROUND OF THE INVENTION

The U.S. Government has certain rights in this invention pursuant toGrant No. N00014-91-J-1174 and N00014-91-J-1452 awarded by the U.S.Navy.

This application is a division of Ser. No. 08/418,287 filed Apr. 7, 1995now U.S. Pat. No. 5,781,648.

1. FIELD OF THE INVENTION

The present invention relates generally to velocity sensors and moreparticularly, to compact integrated electronic circuits with velocitysensors which estimate the speed and direction of a signal stimulusmoving between two photoreceptors or pixels in a one-dimensional or atwo-dimensional on-chip imaging array.

2. BACKGROUND OF THE INVENTION

Various applications in robotic guidance, remote sensing or vehiclecontrol require small, fast sensors for the processing of visual motion.Robust measurement of velocity in real time is difficult but necessaryif a system has to operate in dynamic environments. Parallel processingat each image location is required for handling the large volume ofincoming irradiance data from the optical sensors. Ideally, theprocessed output should neither depend on irradiance nor on contrast.However, because physical systems are always subject to the presence ofnoise, the output is a function of these parameters.

Previous attempts at implementing one-dimensional and two-dimensionalmotion sensors can generally be divided into two main categories,namely, the gradient technique and correspondence technique. Gradientschemes extract the velocity of an image feature from the ratio oftemporal and spatial derivatives of its brightness or discreteapproximations thereof. Correspondence methods measure motion bycomparing the positions of a spatial pattern at different times (spatialcorrespondence), or by comparing the times of occurrence of a temporalpattern at different positions (temporal correspondence). Digitalimplementations of correspondence techniques typically use the firstapproach, where the times of sampling are clocked and the pixeldisplacements of the spatial pattern between these times are variable.Most analog implementations and well-understood biological systems usethe second approach, where the pixel displacements are fixed and thetimes of occurrence of the temporal patterns at those pixels arevariable.

However, existing circuits based on either of these approaches do notprovide a signal that unambiguously encodes velocity, independent ofimage brightness and contrast over typical ranges encountered in naturalscenes. Many of these existing circuits detect edges utilizing apredetermined contrast threshold such that an edge with a contrast levelthat is below the threshold is disregarded, and an edge with a contrastlevel that is above the threshold is assumed to be a sharp edge. Thereare several disadvantages in explicit thresholding. First, bydisregarding images with contrast levels that are below a predeterminedminimum, valuable information regarding the images is lost. Second, thethreshold level has to be changed if the environment or lightingchanges, e.g., if one moves outdoors from an indoor environment. Thus,the scheme is not robust, is sensitive to offsets, and requiresparameter values to be in exactly the right range.

Many of these existing circuits also have an output versus velocitycurve that is not monotonous; the curve has a maximum at some optimalvelocity and decreases on either side of the optimal velocity.Consequently, any given output that is not at the maximum, correspondsto two velocity values and is ambiguous. One such circuit is thatdescribed by T. Delbruck in Silicon Retina with Correlation-Based,Velocity-Tuned Pixels, IEEE TRANS. NEURAL NETWORKS, Vol. 4, p. 529-541,1993.

An early attempt based on the gradient technique is described by J.Tanner and C. Mead, in An Integrated Analog Optical Motion Sensor, VLSISIGNAL PROCESSING 1, 59-76 (S. Y. Kung, Ed., IEEE Press, 1986). Thiscircuit estimates uniform velocity in two dimensions, corresponding toglobal translation of a rigid object space relative to the sensor. Theoutputs of each pixel were made to influence the global estimates of thevelocity vector components in proportion to their deviation from themand to their confidence levels. This strategy was employed to reduceoffset effects of individual pixels by averaging. However, the circuitonly worked with high-contrast edges, and even then showed poorperformance. This result was mainly due to the discrepancy between thehigh-precision requirement of the algorithm and the low precision of theanalog circuitry.

A sensor array based on the correspondence technique is described by T.Horiuchi, J. Lazzaro, A. Moore and C. Koch, in A delay line based motiondetection chip, ADVANCES IN NEURAL INFORMATION PROCESSING SYSTEMS 3,406-12 (Morgan Kaufmnan 1991). In this approach, within each pixel, avoltage pulse was triggered in response to a quickly-increasingirradiance level, identified as a dark-bright edge. Pulses from adjacentpixels were sent through two delay lines from opposite directions. Theirmeeting point, as a measure of their relative timing, was the cue usedto estimate velocity. Thus, each pair of adjacent pixels provided a 1 Dvelocity vector for each detected edge. The circuit worked robustly downto low irradiance levels and contrasts under D.C. lighting conditions.A.C. incandescent lighting, however, caused spurious edges to bedetected at the flicker rate of 120 Hz. This problem could only bealleviated by using additional filtering circuitry. Other drawbacks ofthe system were the limited detectable velocity range for a given delaysetting, and the large area consumption of the delay lines.

A class of chips based on other biologically-inspired correspondencetechniques uses elements tuned to have maximum response to a certainvelocity. In one dimension, such elements usually determine thedirection of non-optimal velocities as well, but they do notunambiguously encode the speed. At least two such cells, tuned todifferent velocities, must be used to extract the speed. In twodimensions, the direction of non-optimal velocity cannot be determinedeither; this is because it is interrelated with the magnitude of thevelocity components measured along different directions.

For a robust estimate of a single velocity, a population of differentlytuned cells is required. Existing systems based on this approach occupylarge silicon areas. In addition, such existing systems exhibit avariety of problems. The velocity-response curve of a chip described byR. G. Benson and T. Delbuck, in Direction Selective Silicon Retina ThatUses Null Inhibition, ADVANCES IN NEURAL INFORMATION PROCESSING SYSTEMS4, 756-763 (Morgan Kaufman 1991), based on a computational model by H.B. Barlow and W. R. Levick, as described in The mechanism ofdirectionally selective units in the rabbit's retina, J. PHYSIOL vol.178, pp. 447-504 (1965), was not tunable and decreased at low contrasts.A second approach by R. Sarpeshkar, W. Bair and C. Koch, as described inVisual Motion Computation in Analog VLSI using Pulses, ADVANCES INNEURAL INFORMATION PROCESSING SYSTEMS 5, 781-788 (Morgan Kaufman 1993)was insensitive to low-contrast edges.

Thus, the sensors in existing systems generally provide velocity outputswhich are strongly dependent upon image brightness or contrast; or theyare inoperable under A.C. incandescent lighting; or they take too mucharea to implement; or they are optimized to detect particular velocitiesand do not unambiguously discriminate between non-optimal velocities; orthey are very sensitive to parameter settings and do not operate overlarge ranges.

Accordingly, there is a need for a velocity sensor which robustly andunambiguously encodes the velocity of a moving stimulus over a largerange of brightnesses, contrasts and velocities, that is compact andinsensitive to offsets and variations in circuit parameters.

BRIEF SUMMARY OF THE INVENTION

An integrated circuit that computes the velocity of a visual stimulusmoving between two photoreceptor locations is disclosed. In its mostbasic version, the circuit comprises two temporal edge detectors withphotoreceptors, two pulse-shaping circuits, and one motion circuit on asingle silicon chip. Velocity is computed from the signed time delay ofthe appearance of an image feature at the two photoreceptor locations.Specifically, each temporal edge detector detects a rapid irradiancetransient at its photoreceptor location and converts it into a shortcurrent spike. This current spike is transformed into two differentvoltage pulses, a fast pulse and a slowly-decaying pulse, by thepulse-shaping circuit that is coupled to the temporal edge detector. Theslowly-decaying voltage pulse produced at one location together with thefast voltage pulse generated at the other location, act as inputs to themotion circuit which generates a signal representative of the speed ofmotion for one sign or direction of motion. A pair of motion circuitsencodes velocity, each motion circuit encoding speed for one of the twoopposing directions of motion. The motion circuits are sample-and-holdcircuits that use the fast pulse from one location to sample theslowly-decaying pulse from the other location. The individualmotion-sensing cells are compact, and are therefore suited for use indense one-dimensional or two-dimensional imaging arrays. Variousembodiments are described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of the architecture of one preferredembodiment of the motion sensor of the present invention.

FIG. 1B is a graph illustrating the outputs of the circuits within themotion sensor of FIG. 1A.

FIG. 2 is a block diagram of the architecture of another preferredembodiment of the motion sensor of the present invention.

FIG. 3 is a block diagram of the architecture of a further preferredembodiment of the motion sensor of the present invention.

FIG. 4A is a block diagram of a preferred embodiment of the edgedetector circuit of the present invention.

FIG. 4B is a detailed schematic diagram of a preferred embodiment of theedge detector circuit of FIG. 4A.

FIG. 4C is a detailed schematic diagram of an alternate embodiment ofthe edge detector circuit of FIG. 4A.

FIG. 5A is a schematic diagram of one embodiment of the pulse-shapingcircuit of the present invention.

FIG. 5B is a schematic diagram of a preferred embodiment of thepulse-shaping circuit of FIG. 5A.

FIG. 5C is a schematic diagram of a second embodiment of thepulse-shaping circuit of the present invention.

FIG. 5D is a schematic diagram of a third embodiment of thepulse-shaping circuit of the present invention.

FIG. 6A is a schematic diagram of a fourth embodiment of the pulseshaping circuit of the present invention.

FIG. 6B is a schematic diagram of a fifth embodiment of thepulse-shaping circuit of the present invention.

FIG. 6C is a schematic diagram of a sixth embodiment of thepulse-shaping circuit of the present invention.

FIG. 7 is a graphical representation of the slowly-decaying outputvoltage V_(s) of the pulse-shaping circuit of the present invention fora variety of edge contrasts.

FIG. 8 is a schematic diagram of a preferred embodiment of the motioncircuit used in the motion sensor of the present invention.

FIG. 9 is a graphical representation of the outputs of the differentstages of the different circuits in the motion sensor of the presentinvention.

FIG. 10 is a graphical representation of the output voltage of themotion sensor of the present invention with respect to velocity for avariety of illumination levels.

FIG. 11 is a graphical representation of the output voltage of themotion sensor of the present invention with respect to velocity for avariety of stimulus types.

FIG. 12 is a graphical representation of the output voltage of themotion sensor of the present invention with respect to the relative timedelay of two voltage pulses presented at the inputs of the pulse-shapingcircuits of FIG. 1A.

CONVENTIONS AND DEFINITIONS

The "grounded-substrate convention" is used herein and described below.According to this convention, a transistor drawn without a bubble on itsgate refers to a native transistor in the semiconductor substrate, andis called a native transistor. A transistor drawn with a bubble on itsgate denotes a transistor in a well, and is known as a well-typetransistor. The semiconductor substrate is always tied to a referencepotential, referred to as "Ground." In an N-well process, the wells aretied to potentials more positive than Ground and the power supply usedin the circuit is more positive than Ground. In a P-well process, thewells are tied to potentials more negative than Ground and the powersupply used in the circuit is more negative than Ground. Utilizing thisconvention, circuits drawn for an N-well process are valid in a P-wellprocess if the signs of all potentials are reversed. This conventionensures that well-substrate junctions are always reverse-biased.

Thus, in an N-well process, well-type transistors are p-channeltransistors in the well, native transistors are n-channel transistors inthe substrate and all voltages in the circuit are positive. In a P-wellprocess, well-type transistors are n-channel transistors in the well,native transistors are p-channel transistors in the substrate and allvoltages in the circuit are negative.

A well-type transistor whose well-terminal connections are notexplicitly described is assumed to have its well-terminal connected tothe power supply.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention involves an elementary sensor for thedetermination of one-dimensional and two-dimensional velocity in realtime. In a preferred embodiment, the elementary sensor comprisesphotoreceptors and analog electronic circuitry on a single siliconsubstrate using VLSI technology. Such analog integrated circuits arecompact and consume little power. Parallel processing of velocity fieldsin real time may be accomplished through the construction of arrays fromthese elementary motion sensors. These sensors are described in detailbelow.

FIG. 1A is a block diagram of the architecture of a preferred embodimentof the motion sensor 10 used to determine one-dimensional,unidirectional velocity in real time. The motion sensor 10 comprises twotemporal edge detectors 12a, 12b, two pulse-shaping circuits 14a, 14band a motion circuit 16. The temporal edge detectors 12a, 12b areidentical. Similarly, the pulse-shaping circuits 14a, 14b are identical.For discussion purposes, the temporal edge detectors 12a and 12b will bereferred to as the temporal edge detector 12. Similarly, thepulse-shaping circuits 14a and 14b will be referred to as thepulse-shaping circuit 14. In a one-dimensional array of motion sensors,each edge detector and pulse-shaping circuit is shared by two adjacentmotion circuits, so that the array can be built of cells comprising oneedge detector, one pulse-shaping circuit and one motion circuit.

The motion sensor 10 responds to a gradient dE in the image irradiance Emoving in one direction of a one-dimensional space, traveling from pixel2 to pixel 1. The gradient dE may include a particular feature of animage such as an edge or a corner. In operation, each temporal edgedetector 12, detects and converts the rapid irradiance transient dE atits location into a short current spike. Each current spike istransformed into two different voltage pulses, a fast pulse V_(f) and aslowly-decaying pulse V₀ s, by the pulse-shaping circuit 14 coupled tothe temporal-edge detector 12, as illustrated in FIG. 1A. In FIG. 1A,the fast and slow pulses corresponding to pixels 1 and 2 are labeledV_(f1), V_(s1) and V_(f2), V_(s2) respectively. Specifically, thepulse-shaping circuit 14a generates a fast pulse V_(f1) and aslowly-decaying pulse V_(s1). Similarly, the pulse-shaping circuit 14bgenerates a fast pulse V_(f2) and a slowly-decaying pulse V_(s2). In thepresent embodiment, the slowly-decaying pulse V_(s2) generated by pulseshaping circuit 14b, together with the fast voltage pulse V_(f1)generated by pulse-shaping circuit 14a, are provided to motion circuit16, which provides an output representative of the velocity of theirradiance gradient traveling from pixel 2 to pixel 1. Theslowly-decaying pulse V_(s1) generated by pulse shaping circuit 14a andthe fast pulse V_(f2) generated by pulse shaping circuit 14b are notutilized in this embodiment. A detailed description of each circuit willbe provided in the following sections.

FIG. 1B is a graph illustrating the voltage outputs of pulse-shapingcircuits 14a, 14b and motion detector 16. The voltage pulse V_(s2) is aslowly-decaying pulse representing the onset and time lapse from thedetection of the irradiance transient dE at pixel 2. As will bedescribed in detail in the following sections, V_(s2) is sampled by themotion circuit 16 when V_(f1), a voltage spike, is received by themotion circuit 16. V_(f1) represents the onset of the detection of theirradiance transient dE at pixel 1. The output of the motion circuit 16,V_(out), which represents the time lapse between the detection of theirradiance transient dE at pixel 2 and the detection of the irradiancetransient dE at pixel 1, is held at the sampled value, until it is resetby another sampling pulse representing the detection of anotherirradiance transient. Since the distance between pixel 1 and pixel 2 isa known value, the time lapse thus obtained will facilitate thecalculation of the velocity of dE from pixel 2 to pixel 1 through theapplication of the formula:

    velocity=distance/time.

FIG. 2 illustrates a block diagram of the architecture of anotherpreferred embodiment of the motion sensor 20 used to determineone-dimensional, bidirectional velocity in real time. The motion sensor20 comprises a pair of temporal edge detectors 12a, 12b, a pair of pulseshaping circuits 14a, 14b and two motion circuits 16a, 16b. The temporaledge detectors 12a, 12b are identical. Similarly, the pulse shapingcircuits 14a, 14b and the motion circuits 16a, 16b are identical. Aone-dimensional array of bi-directional motion sensors can be built ofcells comprising one edge detector, one pulse-shaping circuit and twomotion circuits.

The motion sensor 20 responds to a gradient dE in image irradiance Emoving in either direction of a one-dimensional space; that is, anirradiance transient dE which moves from pixel 1 to pixel 2 orvice-versa. As illustrated in FIG. 2, motion circuit 16a responds to anirradiance gradient traveling from pixel 2 to pixel 1, and motioncircuit 16b responds to an irradiance gradient traveling from pixel 1 topixel 2. As with the embodiment illustrated in FIG. 1A, each temporaledge detector 12 detects and converts the rapid irradiance transient dEat its location into a short current spike. Each current spike istransformed into two different voltage pulses, a fast pulse V_(f) and aslowly-decaying pulse V_(s) by each pulse-shaping circuit 14 coupled toa temporal edge detector 12. Specifically, the pulse-shaping circuit 14agenerates a fast pulse V_(f1) and a slowly-decaying pulse V_(s1).Similarly, the pulse-shaping circuit 14b generates a fast pulse V_(f2)and a slowly-decaying pulse V_(s2). The fast voltage pulse V_(f1)generated by pulse-shaping circuit 14a, together with theslowly-decaying voltage pulse V_(s2) generated by pulse-shaping circuit14b, are provided to motion circuit 16a, which responds to motion frompixel 2 to pixel 1. The fast voltage pulse V_(f2) from pulse shapingcircuit 14b, together with the slowly-decaying voltage pulse V_(s1) frompulse-shaping circuit 14a, are provided to motion circuit 16b, whichresponds to velocity from pixel 1 to pixel 2. Through the use of thepair of motion circuits 16a and 16b, the circuit 20 may compute thevelocity of the irradiance gradient for both directions of motion.

FIG. 3 illustrates a block diagram of the architecture of a furtherpreferred embodiment of the motion sensor 30 used to determinetwo-dimensional velocity in real time. The two-dimensional motion sensor30 comprises two one-dimensional motion sensors 30a and 30b, each ofwhich is identical to motion sensor 20. Motion sensor 30a comprises edgedetectors 12a, 12b, pulse-shaping circuits 14a, 14b and motion detectors16a, 16b. Motion sensor 30b comprises edge detectors 12b, 12c,pulse-shaping circuits 14b, 14c and motion circuits 16c and 16d. Notethat edge detector 12b and pulse-shaping circuit 14b are shared by themotion sensors 30a and 30b. In this embodiment, the photoreceptors usedin each motion sensor all lie in a single plane as illustrated in FIG.3. The circuitry for processing the detected irradiance transientdE--that is, the remaining circuitry in the edge detectors 12a, 12b,12c, the pulse-shaping circuits 14a, 14b, 14c and motion circuits 16a,16b, 16c, 16d may either be external to the photoreceptor plane or lieon the same plane as the photoreceptors. For discussion purposes, anembodiment with the edge detectors 12a, 12b, 12c lying in thephotoreceptor plane and the remaining circuitry lying in differentplanes is described.

For purposes of the following description, a coordinate system isprovided having coordinates oriented as follows: the X and Y axes lie ina horizontal plane as shown in FIG. 3. The Z axis is perpendicular tothe X and Y axes. The edge detectors 12a-12c, pulse-shaping circuits14a-14c and the motion circuits 16a-16d are in planes parallel to thehorizontal plane. In particular, edge detectors 12a, 12b and 12c lie ina first X-Y plane with edge detectors 12a and 12b oriented along the Yaxis and edge detectors 12b and 12c oriented along the X-axis.Similarly, pulse-shaping circuits 14a, 14b and 14c lie in a second X-Yplane parallel to the first X-Y plane, with circuits 14a and 14boriented along the Y-axis, and circuits 14b and 14c oriented along theX-axis. Motion circuits 16a-16d lie in a third X-Y plane parallel to thefirst and second X-Y planes, with motion circuits 16a and 16b orientedalong the Y-axis and motion circuits 16c and 16d oriented along theX-axis.

The motion sensor 30 facilitates the computation of the velocity of anirradiance gradient moving in two dimensions. Motion sensors 30a and 30bwill accurately compute the velocity of an irradiance gradient dE onlyif it is moving along the Y or X axis, respectively. If the gradient ismoving in any 20 other direction, the estimated velocity does notcorrespond to the correct motion. In particular, if an irradiancegradient oriented at an angle Θ to the X axis moves at speed v along itsdirection, the velocities v'_(y) and v'_(x) detected by the motionsensors 30a and 30b respectively are: ##EQU1##

Although a single one-dimensional motion sensor 30a or 30b providesunambiguous encoding of velocity in one dimension, since it providesmonotonic response, it does not account for the orientation of motion ofthe irradiance gradient dE. To uniquely determine orientation and speed,the outputs v'_(y) and v'_(x) of the motion sensors 30a and 30b have tobe combined. The correct speed v may be expressed as: ##EQU2## and thecorrect velocity components v_(x) and v_(y) are given by: ##EQU3## Ifthe photoreceptor pair within each motion sensor 30a, 30b is separatedby the same distance Δd, the measured velocities v'_(x) and v'_(y) aregiven by: ##EQU4## where Δt_(y) and Δt_(x) correspond to the time delayssignaled by the two motion sensors 30a, 30b in the y and in the xdirections respectively.

Thus, ##EQU5##

Thus, utilizing the arrangement illustrated in FIG. 3 permits theunambiguous encoding of the velocity of an irradiance gradient dE movingin two-dimensional space, if the direction of motion is parallel to theorientation of the gradient.

TEMPORAL EDGE DETECTOR

The temporal edge detectors 12a, 12b and 12c are identical and, forpresent purposes, will be referred to as temporal edge detector 12. FIG.4A illustrates a preferred embodiment of a temporal edge detector 12 ofthe present invention. The motion sensor 10, 20, 30a or 30b uses a pairof temporal edge detectors 12 as an input stage. At each photoreceptorsite, a rapid increase in brightness is detected by the temporal edgedetector 12 and converted into a short current spike. In a preferredembodiment, the temporal edge detector 12 detects the temporaldark-bright or ON edges of the image traveling from pixel 1 to pixel 2or vice-versa by sensing voltage changes in the photoreceptor outputcaused by irradiance increases. In an alternate embodiment, the temporaledge detector 12 detects the bright-dark or OFF edges of the imagetraveling from pixel 1 to pixel 2 or vice-versa.

The temporal edge detector 12 comprises an adaptive photoreceptorcircuit (PR) 31 and an amplifier circuit 32 which is basically anoperational amplifier connected to a plurality of transistors andcapacitors so that the amplifier is in a non-inverting feedbackconfiguration. In a preferred embodiment, the temporal edge detector 12of the present invention utilizes the adaptive photoreceptor describedin U.S. Pat. No. 5,376,813 (the "Delbruck patent"), entitled "AdaptivePhotoreceptor including Adaptive element for Long-Time-ConstantContinuous Adaptation with Low Offset and Insensitivity to Light",issued to T. Delbruck and C. Mead and assigned to the assignee of thepresent invention. The Delbruck patent is incorporated by referenceherein. The amplifier circuit 32 comprises an operational amplifier 34,a rectifying circuit (H) 36, a capacitor C5 and an attenuation circuit(A) 38.

FIG. 4B is a detailed schematic of one embodiment of the temporal edgedetector 12 shown in FIG. 4A. In this preferred embodiment, thephotoreceptor circuit 31 comprises photodiode D1, transistors Q1-4 andcapacitors C1, C2. In a preferred embodiment, Q1 and Q2 are n-channeltransistors while Q3 and Q4 are p-channel transistors. However, anyother suitable photoreceptor which can detect irradiance changes in theimage may be used in the edge detector 12.

The photoreceptor described in the Delbruck patent provides an outputvoltage transient dV_(ph) based on a comparison of the actual detectedirradiance and a predicted irradiance based on an average of previousvalues of the detected irradiance. The output voltage V_(ph) of thephotoreceptor 31 increases logarithmically with the irradiance E of thephotodiode D1, as long as the transistors Q1-Q4 operate below threshold.Thus, a transient dE in the image irradiance E causes a voltagetransient dV_(ph) that is a function of the contrast ##EQU6## . Thisproperty is highly desirable for the extraction of local features froman image, because the overall illumination of a typical scene is likelyto change with time, and such a response gives a fixed transient outputfor a given percentage change in irradiance, independent of the currentoverall brightness of the image. When there is steady illumination onD1, the bias voltage V_(bp) biases Q3 as a current source, which pullsthe voltage of node A up, so as to turn on Q1 through diode-connectedQ4, thereby pulling the voltage on node B up. As a result, Q2 starts toturn on until the current passing through Q2 equals the current throughQ3, a constant current. Thus, at steady state, the voltage of node B isfixed, i.e., independent of the irradiance E. In the photoreceptorcircuit 31, the DC gain is low and the transient gain is high. Thetransient gain of the photoreceptor is determined by the capacitivedivider formed by C1 and C2.

When a sudden increase in illumination is detected by D1, the current ofD1 increases proportionally. The operating point of the photoreceptorcircuit 31 is set by a previous average value of the irradiance. Thus,the input photocurrent of the photoreceptor circuit 31 at this pointcomprises a steady-state background component I_(bg) and a varying ortransient component i. The transient component i pulls node B down,which reduces the current through Q2. This causes the voltage of node Ato increase with high gain, which in turn increases V_(q) through thecapacitive divider formed by C1 and C2. Due to the high gain of thisfeedback circuit, V_(p) is held nearly clamped and V_(q) rises enough tomake Q1 provide the extra current i sunk by D1. Consequently, V_(ph)rises rapidly, then slowly adapts towards the voltage V_(q) as C1 ischarged through the adaptive element Q4. For typical irradiances, Q1operates at subthreshold and thus the voltage V_(q) increaseslogarithmically with the irradiance of the photodiode D1.

The current-voltage (I-V) relationship of the adaptive element Q₄ isthat of a sinh. Consequently, the adaptation is slow for small outputvoltage steps and fast for large steps. The adapted gain of thephotoreceptor circuit 31 is low because the feedback is a short circuitacross the adaptive element Q4 and V_(ph) does not have to adjust by asignificant amount to cause the necessary current increase through Q1.On short time scales, no charge flows through the adaptive element Q4,but changes in V_(ph) are coupled to V_(q) through the capacitivedivider comprising C1 and C2. Thus, the larger the capacitive dividerratio, the larger V_(ph) will become to move V_(q) to account for theincreased current through Q1 and D1. Thus, the transient gain of thephotoreceptor 31 is set by the capacitive-divider ratio.

In subthreshold operation, the transient change dV_(ph) of thephotoreceptor output voltage to an irradiance step dE is given by##EQU7## where ##EQU8## is the thermal voltage and K is the back-gatecoefficient of Q₁.

As depicted in FIGS. 4A and 4B, the output of the photoreceptor 31 isfed into an amplifier circuit 32 that transduces positive voltageexcursions in the photoreceptor voltage V_(ph), corresponding to ONedges, to a current. The circuit 32 comprises an operational amplifier34 with a bias V_(b), connected to a rectification circuit 36, acapacitor C5, and an attenuation circuit 38 so that the amplifiercircuit 32 is in a non-inverting feedback configuration.

In a preferred embodiment, the operational amplifier 34 is a wide-rangeamplifier comprising transistors Q₅ -Q₁₃. When no irradiance transientis detected by the photoreceptor 31, dV_(ph) =0 and the non-invertingand the inverting terminals, nodes 2 and 1, of the operational amplifier34 are at the same potential. V_(b) biases Q₅ to provide a currentsource for the differential input stage of the operational amplifier 34,turning on Q₆ and Q₇ with an equal amount of current. Q8 and Q9 act ascurrent mirrors, mirroring the current passing through Q₆ and Q₇ to Q₁₀and Q₁₁ respectively. The current through Q₁₀ is mirrored at Q₁₂ to Q₁₃.Since the current in Q₁₃ is equal to that in Q₁₁, the potential atV_(amp) is steady.

When an irradiance transient dE is detected by the photoreceptor 31,dV_(ph) becomes nonzero. In this case, nodes 1 and 2 are not at the samevoltage. Specifically, if dV_(ph) is positive, node 2 will temporarilybe more positive than node 1. The current passing through Q₆ willincrease and the current passing through Q₇ will decrease. This causesmore current to be mirrored by Q₈ to Q₁₀, resulting in more currentbeing mirrored by Q₁₂ to Q₁₃. In addition, less current will be mirroredthrough Q₉ to Q₁₁. As a result, the current through Q₁₃ is greater thanthe current through Q₁₁ and the potential at Vamp will rise sharply.

If the polarity of dV_(ph) is reversed, the potential at node 2 willtemporarily be less than the potential at node 1. As a result, morecurrent will pass through Q₇ than when nodes 1 and 2 were at the samepotential. This increased current is mirrored by Q₉ to Q₁₁. In addition,less current will pass through Q₆, which is mirrored by Q₈ to Q₁₀. Thedecreased current will be mirrored by Q₁₂ to Q₁₃. As a result, thecurrent through Q₁₁ will be greater than the current through Q₁₃, andthe potential at V_(amp) will sharply decrease.

The output of the operational amplifier 34 is provided to therectification circuit 36, which is in turn coupled to the attenuationcircuit 38. The rectification circuit 36 comprises transistors Q₁₅, Q₁₆and Q₁₇. The attenuation circuit 38 comprises capacitors C₃, C₄ and anadaptive element Q₁₄. The adaptive element Q₁₄ is a sinh element,identical to Q₄ used in the photoreceptor 31. The adaptive element Q₁₄prevents node 1 of the amplifier 34 from floating by slowly adapting itto V_(int). The capacitors C₃, C₄ form a capacitive divider that causesthe transient gain at V_(int) in the amplifier circuit 32 to be ##EQU9##with respect to the input voltage change dV_(ph) of the amplifier 34.

The current I charging the node V_(int) is given by ##EQU10## providedthat Q₁₄ is only weakly turned on. This current is supplied by Q₁₆ if itis positive and by Q₁₅ if it is negative. Thus, if the current througheach of these transistors is sensed separately, a half-wave rectifiedversion of the changing current I may be constructed. In this preferredembodiment, the positive part of I is sensed by sensing the currentthrough Q₁₆ with the diode-connected Q₁₇. The voltage V_(out) on thegate of Q₁₇ may be used to mirror copies of the current through Q₁₆ tosucceeding circuits. The voltage V_(g), which provides a bias for Q₁₇,may be used to set the gain of this output mirror if it is operatedwithin a few mV of V_(DD). Note that the transistor Q₁₅ issource-connected to minimize body-effects, since the D.C. voltages ofoperation are around 1.5 V. Also note that FIG. 4C is a detailedschematic diagram of an alternate embodiment of the edge detectorcircuit of FIG. 4A. Specifically, FIG. 4C illustrates one implementationof the rectifying circuit 36a where Q15a and Q17a are native transistorsand Q16a is a well-type transistor.

Substituting equation (1) into equation (2) provides an output currentof ##EQU11## The output current is therefore proportional to thetemporal contrast ##EQU12## where the temporal contrast is the productof the velocity v and the spatial contrast ##EQU13##

Thus, the amplifier circuit 32 senses a voltage change ##EQU14## in thephotoreceptor 31 output caused by a detected temporal contrast ##EQU15##and serves to convert the sensed voltage change into a current Ilinearly proportional to the detected temporal contrast by amplifying,differentiating, and rectifying the received signal ##EQU16##

PULSE-SHAPING CIRCUIT

As described earlier, the pulse-shaping circuits 14a, 14b and 14c areidentical. For present purposes, each pulse shaping circuit 14a, 14b and14c will be referred to as a pulse-shaping circuit 14. FIG. 5A is aschematic diagram of one embodiment of the pulse shaping circuit 14 ofthe present invention. This preferred embodiment of circuit 14 is anon-linear differentiator comprising a low-pass filter 40 in thefeedback path of a high-gain amplifier 42. The high gain amplifier 42comprises transistors Q₁₈ and Q₁₉ and the low pass filter 40 comprisestransistor Q₂₀, exponential elements EX1, EX2 and capacitor C. FIG. 5Bis a schematic diagram of a preferred embodiment of the pulse-shapingcircuit of FIG. 5A. In this preferred embodiment, exponential elementEX1 is a native transistor Q₂₁ in diode configuration and exponentialelement EX2 is an identical native transistor Q₂₂, in diodeconfiguration. Note that each exponential element can be implementeddifferently, for example, as a npn bipolar transistor in diodeconfiguration.

The input to the circuit 14 is a current I_(in), obtained from themirror constructed by connecting the output voltage of the edge detector12 (V_(out) in FIG. 4B) to V_(in) of circuit 14. The voltage V_(f)responds to an input current spike I_(in), mirrored from the edgedetector 12, with a voltage spike. The voltage V_(s) responds to thesame spike with a pulse having a sharp onset and a log(t)-like decay.The input I_(in) may be thought of as an impulse that sets the initialcondition on the diode-capacitor subcircuit of Q₂₁, Q₂₂, and C. It maybe shown that, for an initial condition with a spike height of I_(o),the diode-capacitor current I_(out) is given by ##EQU17## where##EQU18## and K is the back-gate coefficient of Q₂₁ and Q₂₂.

Specifically, I_(in) is a current spike which turns on Q₁₈, pulling thevoltage V_(f) up and turning on Q₂₀. This causes capacitor C to becharged rapidly by the current through Q₂₀. As C charges up, the drainvoltage of Q₂₁ increases, thereby turning on Q₁₉ until its currentbalances the current flowing through Q₁₈. When the input-current spikeceases, the voltage V_(f) falls abruptly because the current through Q₁₉is now greater than that through Q₁₈ and Q₂₀ is turned off. Thecapacitor C then discharges through Q₂₂ and Q₂₁, resulting in a slowdecay of voltage V_(s).

FIG. 5C is a schematic diagram of a second embodiment of thepulse-shaping circuit of the present invention. The circuit 14 of FIG.5C is substantially identical to the circuit of FIG. 5A, with theexception that the capacitor C is coupled to the gate of Q₁₉ instead ofbeing coupled to the source of Q₂₀.

FIG. 5D is a schematic diagram of a third embodiment of thepulse-shaping circuit of the present invention. The circuit of FIG. 5Dis substantially identical to the circuit of FIG. 5A, with the exceptionthat exponential element EX2 is not utilized in the circuit of FIG. 5D,so that the source of Q₂₀ is connected directly to the exponentialelement EX1.

FIGS. 6A-6C are schematic diagrams of a fourth, a fifth and a sixthembodiment respectively, of the pulse-shaping circuit of the presentinvention. The circuit 15 illustrated in FIGS. 6A-6C correspond to thecircuit 14 illustrated in FIGS. 5A, 5C and 5D, respectively, but nativeand well-type transistors are exchanged, V_(DD) and Ground are exchangedand different exponential elements EX3 and EX4 are used. In a preferredembodiment, each exponential element is either a well-type MOSFET indiode configuration or a pnp bipolar transistor in diode configuration.The input voltage V_(in) of circuit 15 is the output voltage V_(out) ofthe embodiment of edge detector 12 depicted in FIG. 4C.

Note that the circuit 14 or 15 has no explicit time constant, determinedby a bias voltage, that sets the dynamics of the diode-capacitor decay.Instead, it exploits the fact that a diode-capacitor configurationintrinsically adapts to time constants over many orders of magnitude.After a sufficiently long time t, such that I_(o) t>>CV_(K), I_(out) (t)is approximately equal to CV_(K) /t as seen from equation (3), and V_(s)(t) is approximately proportional to V_(K) log(t). This means thatI_(out) and V_(s) are independent of I_(o). Because I_(out) and V_(s)are independent of I_(o) if it is sufficiently large, i.e., if the inputtemporal contrast dE/Edt is sufficiently large, the shape of V_(s) (t)becomes invariant to input contrast level at times when it will besampled. This can be seen from FIG. 7, which depicts V_(s) (t) inresponse to edges of four different contrasts. Thus, the sample-and-holdmotion circuits, to be described later, that have V_(s) (t) as theirinput report velocities that do not change with input contrast so longas it is sufficiently large. This contrast-independence is achievedwithout any explicit digital thresholding, as in several other schemes,because of the use of the graceful analog properties of the circuit.Also, because of the intensity-independent encoding of thephotoreceptor, the outputs are independent of light level as well.

MOTION CIRCUITS

FIG. 8 depicts a schematic diagram of a preferred embodiment of themotion circuit 16 of the present invention. To compute the velocity ofan irradiance gradient dE, the analog voltage of the slowly-decayingpulse V_(s) that the irradiance gradient dE initiates at one pixel issampled by the voltage spike V_(f) triggered by the detection of thesame irradiance gradient dE at an adjacent pixel. Since the V_(s) pulsefacilitates the measurement, this technique is termedfacilitate-and-sample (FS). The monotonic decay of the facilitationpulse ensures unambiguous encoding of speed in the sampled voltage. Forthe determination of a signed velocity component (i.e., the velocity intwo opposite directions), two sample-and-hold circuits are necessary;this is so because each circuit can only determine speed for thedirection of motion where the V_(s) pulse is initiated before the V_(f)spike. This direction is called the preferred direction.

For example, if dE travels from pixel 2 to pixel 1 (see FIG. 1A),pulse-shaping circuit 14b generates V_(s2) when the photodiode D1 inedge detector 12b senses dE at pixel 2. Pulse-shaping circuit 14agenerates V_(f1) when the photodiode in edge detector 12a senses thearrival of dE at pixel 1. The output voltage V_(out) of motion circuit16 is then representative of the speed of dE. In the other direction,called the null direction (i.e., from pixel 1 to pixel 2 for motioncircuit 16), the sampling pulse precedes the facilitation pulse and thevoltage of the facilitation pulse triggered by the previous edge issampled. The latter voltage is normally low unless edges arrive in quicksuccession. Pulse-shaping circuits 14a, 14b also generate V_(s1) andV_(f2) respectively, which are intended for use by motion circuit 16b(see FIG. 2) for measuring the speed of dE, when dE travels from pixel 1to pixel 2.

Specifically, in FIG. 8, V_(bs) biases Q₂₃ to provide a current sourcefor the motion circuit 16. The current through Q₂₃ divides at node 3 andat steady state passes equally through Q₂₄ and Q₂₅. Q₂₆ mirrors thecurrent through Q₂₄ to Q₂₇. Since the current through Q₂₅ is equal tothe current through Q₂₇ and thus through Q₂₆ and Q₂₄, the voltage on thegate of Q₂₅ is equal to that on the gate of Q₂₄. Thus, the voltageV_(s2) on the gate of Q₂₄ is effectively copied to node 4 by the buffercomprised of transistors Q₂₃ -Q₂₇

When V_(f1) is provided to the motion circuit 16, Q₂₈ turns on andV_(f1) effectively samples V_(s2) a period of time after V_(s2) isprovided to the motion circuit 16. This results in an output voltageV_(out) that is equal to the amplitude of V_(s2) at the sampling time,and specifically, at the time dE is detected at pixel 1.

Motion circuits 16a, 16b, 16c, and 16d are identical. However, themotion circuit 16a responds to motion from pixel 2 to pixel 1 in FIG. 2and the motion circuit 16b responds to motion from pixel 1 to pixel 2.With reference to FIG. 2, motion circuit 16a receives V_(s2) frompulse-shaping circuit 14b located at pixel 2 and V_(f1) frompulse-shaping circuit 14a located at pixel 1. In contrast, motioncircuit 16b receives V_(s1) from pulse-shaping circuit 14a located atpixel 1 and V_(f2) from pulse shaping circuit 14b located at pixel 2.Similarly, motion circuit 16c receives the slowly-decaying pulse frompulse-shaping circuit 14c and the fast pulse from pulse-shaping circuit14b, and motion circuit 16d receives the slowly-decaying pulse frompulse-shaping circuit 14b and the fast pulse from pulse-shaping circuit14c.

The motion circuits 16a-16d (referred to in general as motion circuit16) respond down to arbitrarily slow speeds, while showing goodsensitivity at high speeds. Under the assumption that ##EQU19## >>CV_(K)where Δd is the pixel spacing, the sampled output voltage V_(out) isindependent of I_(o) and is proportional to V_(K) log ##EQU20## . Thesensitivity ##EQU21## is highest at slow speeds, decaying with v⁻¹. Asingle element in a one-dimensional array of motion sensors consistingof a temporal edge detector, a pulse-shaping circuit, and two motioncircuits comprises 34 transistors, if it uses the preferred embodimentsshown in FIG. 4B, FIG. 5B and FIG. 8.

EXPERIMENTAL RESULTS

A motion sensor 20 was fabricated using a 2 μm n-well CMOS process. Asingle element in a one-dimensional array of such motion sensors coversan area of 0.05 mm². The imaging lens used for circuit testing has afocal length f=13 mm and an f-number of 1.8. For quantitativemeasurements, sheets of paper with printed gray scale patterns werewrapped around a rotating drum to provide the optical stimuli. Theobject distance was set to 380 mm. Measurements were taken underincandescent room lighting conditions, where a white paper surfaceprovided an illuminance of about 1.2 lux on the circuit plane. FIG. 9shows the response of the different stages of the circuit to a blackmoving bar on a white background. The 120 Hz flicker noise is seen onthe output voltage trace V_(amp) of the wide-range amplifier (FIG. 4B).The bias of the operational amplifier V_(b) was set at low gain so thatthe flicker noise remained tolerable. With the bar moving from pixel 2to pixel 1, V_(s2) is first generated followed by V_(f1), and the outputvoltage V_(out) is obtained by sampling V_(s2) when V_(f1) is triggered.

The response curves for the preferred direction at different globalillumination levels are shown in FIG. 10. The proximity of the curves toeach other is a result of the good contrast encoding of thephotoreceptor. Robust operation was observed down to very dim roomillumination levels. In FIG. 11, the effect of using different edgesharpnesses and contrasts under standard room lighting conditions isshown. The decrease of the response at relatively high contrasts is dueto the fact that the input stage had to be operated at low gain in orderto suppress 120 Hz flicker noise. Experiments with D.C. illuminationshow that a 40% contrast sinusoidal stimulus can be made to yield almostthe same response as a 100% contrast bar stimulus for sufficiently slowspeeds.

FIG. 12 shows the response of the motion circuit if the pulse-shapingcircuits receive normalized, electronically generated voltage pulses asinputs (V_(in) in FIG. 5B) instead of the output pulses of the edgedetectors. The output voltage is plotted as a function of the relativetime delay between two pulses applied to adjacent pulse-shapingcircuits. Note that the time axis is on a log scale. For such idealinputs, the logarithmic dependence of the output signal on the relativetime delay is approximately maintained for time delays spanning sixorders of magnitude. Monotonic dependence of output voltage on timedelay is observed over eight orders of magnitude.

Thus, it can be observed that the motion circuit of the presentinvention provides a velocity output that is independent of thebrightness and contrast of the image over considerable ranges. Themotion circuit of the present invention can also unambiguously encodethe velocity of a moving stimulus. In addition, the motion circuitoperates robustly over large speed ranges and is also more compact, andresponds to lower contrasts than other known implementations.

Modifications and variations of the embodiments described above may bemade by those skilled in the art while remaining within the true scopeand spirit of this invention. Thus, although the present invention hasbeen described in terms of certain preferred embodiments, otherembodiments that will be apparent to those of ordinary skill in the artare also within the scope of this invention. Accordingly, the scope ofthe invention is intended to be defined only by the claims which follow.

What is claimed is:
 1. A circuit for detecting the presence of an imagefeature having variable brightness, comprising:a photoreceptor forsensing the brightness distribution of the image feature and forproviding a first signal in response; and an amplifier circuit coupledto the photoreceptor for amplifying the first signal and providing asecond signal representative of the variable brightness, the amplifiercircuit comprising:an operational amplifier; a rectifying elementcoupled to an output of the operational amplifier; an attenuationcircuit coupled to the rectifying element to provide a feedback path tothe operational amplifier; and a capacitive element coupled between anoutput of the rectifying element and a power supply terminal.
 2. Thecircuit of claim 1, wherein the rectifying element comprises:first,second and third transistors, each having a source, a drain and a gate,the first and third transistors being of a first conductivity type andthe second transistor being of a second conductivity type, the drain ofthe first transistor being coupled to its gate and to the drain of thesecond transistor, the source of the second transistor being coupled tothe source of the third transistor and to the capacitive element, andthe gates of the second and third transistors being coupled together andto the output of the operational amplifier.
 3. The circuit of claim 2wherein the first conductivity type is well-type and the secondconductivity type is native.
 4. The circuit of claim 2, wherein thefirst conductivity type is native and the second conductivity type iswell-type.
 5. The circuit of claim 1, wherein the attenuation circuitcomprises first and second capacitors and a well-type transistor havinga source, a drain, a gate, and a well, the first capacitor being coupledto the source and well of the transistor at one end and being coupled tothe second capacitor and the drain and gate of the transistor at theother end.
 6. The circuit of claim 1, wherein the image feature is anedge of an image.